Why does my shower run intermittently cold?

I have a mixer shower connected to a combi boiler (the model is RD628) and the water runs intermittently completely cold for approx 15 seconds approx every 2 mins. When I look at the boiler the ignition light is going on and off. The fault light is not turning which suggests its not an overheat cut off issue.

Every other tap in my home doesn't have this problem, I can run the hot water constantly and the temperature is consistent, and the ignition light on the boiler remains constant. This is also true for the shower providing I turn the water up to be very hot.

I have already tried changing the thermostatic value in the shower and this did not resolve issue, and I bought a completely new bar shower as well and this also did not resolve the issue.

I have tried changing the hose and the shower head also. I have been living with this for 7 years and am getting a bit sick of it.

Please help, even if the just advise on what sort of professional to engage. Is this a plumbing problem or a boiler problem?